1. Field of the Invention
The present invention relates to polarizing glass articles suitable for use in microisolators which are applicable in optical communication utilizing semiconductor lasers and optical fibers.
2. Related Art
Polarizing glasses which contain uniaxially oriented metallic particles having a large aspect ratio dispersed therein have already been known. Japanese Patent Application Laid Open JP-A-! No. 56-169140 (hereinafter referred to as "prior art 1"), for example, discloses a process for the production of a polarizing glass, which comprises the steps of stretching a glass containing silver halide particles such as AgCl, AgBr or AgI, and then reducing the stretched glass in a reducing atmosphere of 300.degree. C. or more, whereby uniaxially elongated metal silver particles are precipitated to afford a polarizing glass.
Japanese Patent Application Laid Open JP-A-! No. 5-208844 (hereinafter referred to as "prior art 2") discloses a polarizing glass and the production process thereof, which comprises bringing about the precipitation of copper halide particles such as CuCl, CuBr or CuI in the glass, stretching and reducing the resultant glass to obtain a polarizing glass which contains uniaxially stretched metallic copper particles precipitated therein.
Japanese Patent Application Laid Open JP-A-! No. 59-83951 (hereinafter referred to as "prior art 3") discloses a polarizing glass and the production process thereof, according to which, in order to avoid the glass to break due to concentrated tensile stress near the glass surface, a potential polarizing glass which contains unprecipitated and unstretched metallic particles and is called "core glass" is coated with a glass having much lower viscosity, whereby obtaining a photochromic polarizing glass in which tensile stress is scarcely present near the surface of the stretched article.
The polarizing glasses disclosed in prior art 1 and prior art 2 are produced by stretching a metallic halide-containing glass and then maintaining the stretched glass at an elevated temperature in a reducing atmosphere to precipitate metallic particles. In such processes, however, in the inner part of the glass, metallic halide particles remain almost in the unreduced state and consequently the precipitated metallic particles are present only in the surface layer having an extremely small thickness. Metallic halides remained in the glass give no contribution to polarizing properties of the glass. On the contrary, metallic halide particles, since having a refractive index different from that of base glass therearound, may cause light scattering and hence transmission loss due to the scattering of a part of incident light.
Moreover, metallic halide is partially ionized in the glass. Ionized metals absorb the light of specific wavelength ranges and this may cause additional transmission loss depending on the wavelength range.
Prior art 3 discloses a laminated polarizing glass. The invention of this prior art is directed to a polarizing glass having photochromic properties. Core glasses which can be used are only potentially polarizing glasses which contain unprecipitated and unstretched metallic particles and hence inevitably contain metallic halide particles. Consequently, light scattering occurs due to the difference in refractive index between the base glass and the metallic halide particles, which results in the transmission loss thereof.
Further, to avoid the break occurring due to the presence of tensile stress near the surface, the core glass is coated with a glass having a much lower viscosity. Thus, one of the characteristics of such glasses is that the coating glass is different in composition from the core glass. However, when the coated glass, i.e., surface glass has a composition different from that of a core glass, both glasses are generally differ in refractive index and the reflection loss occurs at the interface of the both glasses due to the difference in refractive index therebetween. This is another factor which may bring out the transmission loss of light.
As explained above, conventional polarizing glasses exhibit a large transmission loss, which is an important problem from the viewpoint of practical use. In particular, when a polarizing glass is to be applied to optical isolators, the transmission loss of a polarizing glass is responsible for the greater part of the total transmission loss of the isolators.
In the polarizing glasses described in the above-cited prior arts, it would be possible to diminish to some degree the transmission loss by applying a longer reduction time, but a satisfactory level of the transmission loss could not be obtained. Further, the longer reduction time may lead to a lower productivity and thus is scarcely practicable.
On the other hand, by making the total thickness of a polarizing glass smaller, a thinner layer containing metallic halide particles may be produced, thus enabling to diminish to some degree the transmission loss. Thinner polarizing glass may, however, be poorer in resistance to stress and therefore it may become more difficult to handle such glass and thus it is impracticable. In addition, while ordinary polarizing glass should be polished to improve the precision of the surface, a thinner glass may be disadvantageously unpolishable because its deformation may occur when placed on a polisher plate. As examined by the present inventors, when a polarizing glass disclosed in prior art 2 is imparted with a thickness of 0.3 mm instead of 1 mm, the glass may achieve the diminish of the transmission loss by about 2% while maintaining polarizing properties thereof. Such glass, however, did not have sufficient strength and thus is impracticable.
Therefore, an object of the present invention is to provide a polarizing glass having polarizing properties comparable to conventional counterparts, a largely decreased transmission loss and a strength acceptable to practical use as well as the production process thereof.